Exploring the potential impacts of global change on the woody component of of South African savannas
Doctoral Thesis
2014
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University of Cape Town
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Land cover change, elevated CO2 and associated climate change are driving changes in ecosystem structure and function across the world. These changes are best described and understood in northern hemisphere temperate systems. Studies, models and reports from these regions are therefore often used as the basis for understanding, predicting and managing change in other biomes, including African savannas. In savannas elevated CO2 has been implicated in driving an increase in woody cover, however attribution to global drivers is often confounded by land - use management which is also implicated as a driver of tree cover increases. Climate change is predicted to directly drive species range shifts in savannas, however several important gaps exist in the literature which prevent the development of a clear predictive framework to describe these changes. Whilst climate is often assumed to be the primary factor underlying savanna tree distribution this has not been demonstrated for savanna tree species. Additionally the determinants of individual savanna species distributions have seldom been investigated. The few existing studies are correlative species distribution models based on adult plant distribution and underlain by the assumption that climate sets plant distribution. Although a demographic approach is very important in understanding tree: grass coexistence in savannas, this approach is seldom used in understanding species distributions. The aim of this study was therefore to assess how global change might influence South African savannas. We place emphasis on the role of elevated CO2 and climate change. I first report on an analysis of historical changes in woody plant abundance at large spatial scales to assess the magnitude of tree cover change in South African savannas. I attempt to untangle the role of local drivers and global drivers in causing tree cover changes from 1940-2009. I then report on studies aimed at determining what limits the distribution range of two common savanna trees Acacia nigrescens and Colophospermum mopane, at three critical demographic stages. Here I used a series of field studies, transplant experiments, lab experiments and modelling to determine the critical drivers of the distribution of these plants so as to understand how savanna plants might respond to changing climates.
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Stevens, N. 2014. Exploring the potential impacts of global change on the woody component of of South African savannas. University of Cape Town.